In order to generate a rotating magnetic field depending on an electrical angle in a rotating body such as a brushless motor, for example, the rotational position (rotation angle) of a rotor is detected by using typically a resolver, and a control unit drives a three-phase bridge circuit based on a detection signal generated by the resolver. In accordance with this method, the output of the resolver needs to be converted into a position signal by using typically a special R/D converter (conversion unit for converting a resolver signal into a digital signal).
However, the use of the R/D converter, which is larger and more expensive than other components, increases the size and manufacturing cost of an ECU (electronic control unit). In order to solve this problem, the ECU is provided with a microcomputer for outputting an exciting signal to the resolver and finding a rotation angle based on a signal, which is output by the resolver in accordance with the exciting signal.
In addition, the ECU is also provided with an input/output circuit for facilitating the computation of the rotation angle by the microcomputer. Then, the rotation angle is computed (patent document 1) or the rotation angle is corrected (patent document 2) by using input and output signals of the input/output circuit.
[Patent Document 1] Japanese Patent Laid-open No. 2003-166803, which corresponds to US 20030111974A1
[Patent Document 2] Japanese Patent Laid-open No. 2001-264114
In accordance with the apparatus in patent document 1, in the event of a failure caused by a short circuit between a terminal outputting an exciting signal to a resolver (such a terminal is referred to hereafter as an exciting-signal output terminal) or a signal line and a power supply (such a failure is referred to hereafter as a power-supply short-circuit failure) or in the event of a failure caused by a short circuit between the exciting-signal output terminal or the signal line and the ground (such a failure is referred to hereafter as a ground short-circuit failure), it is likely that a current excessively greater than a current flowing at a normal time continues to flow to a circuit connected to the exciting-signal output terminal.
In order to solve the above problem, when an abnormality of the exciting-signal output terminal is detected, only power supplied to an actuator driven and controlled by the ECU is cut off while an IG switch for supplying power to the ECU is kept in an ON state in order to prevent the system from being stopped due to discontinuation of an operation of power supply to the ECU. In consequence, however, the exciting signal is generated as it is. As a result, it is found that, since the excessively large current continues to flow to the circuit connected to the exciting-signal output terminal from a power supply by way of the ECU, internal components in the circuit overheats and fails.